It is known that exposure of human or animal tissue to ionizing radiation will kill the cells thus exposed. This finds application in the treatment of pathological cells, for example. In order to treat tumors deep within the body of the patient, the radiation must first penetrate the healthy tissue in order to irradiate and destroy the pathological cells. In conventional radiation therapy, large volumes of healthy tissue can thus be exposed to harmful doses of radiation, resulting in prolonged recovery periods for the patient. It is therefore desirable to design a device for treating a patient with ionizing radiation and treatment protocols so as to expose the pathological tissue to a dose of radiation which will result in the death of those cells, whilst keeping the exposure of healthy tissue to a minimum.
Several methods have previously been employed to achieve the desired pathological cell-destroying exposure whilst keeping the exposure of healthy cells to a minimum. Many methods work by directing radiation at a tumor from a number of directions, either simultaneously from multiple sources or multiple exposures from a single source. The intensity of radiation emanating from each direction is therefore less than would be required to actually destroy cells (although still sufficient to damage the cells), but where the radiation beams from the multiple directions converge, the intensity of radiation is sufficient to deliver a therapeutic dose. By providing radiation from multiple directions, the amount of radiation delivered to surrounding healthy cells can be minimized. Of course it is also important that the radiation should be accurately targeted on the region that requires treatment. For this reason, patients are required to remain still for the duration of the therapy session, to minimize the risk of damage to healthy tissue surrounding the target region. However, some movement is inevitable, e.g. through breathing, or other involuntary movements. To overcome this problem, it is known to integrate an image acquisition system with the radiotherapy apparatus, to provide real-time imaging of the region (i.e. surveillance) and ensure that the radiation emitted by the radiotherapy apparatus is not misdirected.
One such integrated image acquisition system is a computed tomography (CT) scanner. In these systems, a source of kV radiation emits a cone beam of radiation towards the patient, with the scattered radiation being detected by an imager positioned substantially opposite the source. The projection images, acquired at a range of angles around the patient, can then be reconstructed using known techniques to provide a three-dimensional image of the region undergoing therapy. In addition, single projection images can be used to detect the instantaneous position of the patient anatomy during radiotherapy.
Currently the kV projection images required for a 3D volume/CBCT scan are acquired at a constant rate as the gantry rotates around the isocenter during treatment. For example, kV projection images required for surveillance, i.e. ensuring the patient has not moved during treatment, may be acquired at the readout rate of the detector.